The present invention relates in general to communication systems and circuits therefor, and is particularly directed to the use of a modified alarm fuse receptacle wired as a ringing voltage switch, so as to selectively provide connectivity for a ringing voltage circuit, in a manner that is effective to protect maintenance personnel servicing telephone equipment from a ringing voltage shock hazard.
The operation of the ringing circuit for a standard (plain old telephone service, or POTS) analog telephone installed at a subscriber premises typically requires the application of a ringing voltage of at least 40 Vrms to the customer""s handset. As diagrammatically illustrated in FIG. 1, the ringing voltage is customarily generated remotely by a ringing voltage generator 11 for a line card 13 installed in a channel bank of a central office (CO) 10. Because there can be a significant (voltage dropping) distance over the metallic span 21 between the source of the ringing voltage (the line card 13) and the subscriber""s handset 23, the ringing voltage generated at the central office will normally be much higher than 40 Vrms. As a non-limiting example, the ringing voltage at the line card 13 may be on the order of 105 Vrms. This substantially elevated voltage produced by the ringing voltage generator poses a shock hazard to service personnel that must be avoided during equipment maintenance (e.g., adding or removing line cards in the channel bank, etc). Moreover, it is preferable to disable only the ringing voltage generator during these times, so that non-ringing POTS lines remain functional.
In accordance with the present invention, this objective of protecting telephone equipment maintenance personnel from a potentially hazardous ringing voltage, while allowing other POTS functionality to be operational, is successfully achieved by installing a three-terminal alarm fuse socket or receptacle in a circuit path for the ringing voltage generator, and wiring the fuse socket""s terminals to operate as a switch. The switch is closed by inserting a three-pin fuse (which may either blown or unblown) into the socket, so as to enable a ringing voltage generated by the ringing voltage generator to be applied to the span and delivered thereby to the ringing signal circuit of the handset. The switch is opened by simply removing the fuse. Thus, ringing voltage functionality for and/or connectivity with the ringing voltage circuit is dependent upon whether a fuse is installed in the socket, regardless of the state of the fuse.
The fuse socket itself has a first terminal at one end thereof connected to a first location of an operation-enabling current flow path of the ringing voltage generator. Second and third terminals at a second end of the socket are coupled in common to a second location of the ringing generator""s current flow path in which the alarm fuse socket is installed. The fuse receptacle is sized to receive a three-terminal, or three-pin fuse cartridge, commonly referred to as an alarm indicating fuse, having a source electrode, a load electrode and an alarm electrode. The fuse cartridge has a flexible, tensioned conductor affixed to the source electrode and normally biased against and held by a fusible element adjoining the load electrode.
With this three-pin configuration, the alarm fuse normally provides electrical connectivity between its source and load electrodes. For an overcurrent condition, however, the fusible element is severedxe2x80x94blowing the fusexe2x80x94causing the tensioned conductor to flex away from the load electrode and into physical and electrical contact with the alarm electrode. In a customary three-terminal alarm fuse installation, the alarm terminal of the receptacle is connected to an alarm output, so that when the fuse blows, an alarm indication may be generated.
According to the invention, however, since the load and alarm terminals of the fuse socket are wired in common, it makes no difference whether an xe2x80x98unblownxe2x80x99 fuse or a xe2x80x98blownxe2x80x99 fuse is installed. In either case, a closed electrical path is provided through the fuse and thereby through the operation-enabling current flow path of the ringing voltage generator. Namely, wiring the socket so that the alarm and load pins of the fuse are effectively electrically shorted together forces a connection between the source and load pins regardless of the state of the fusing element (as long as the fuse cartridge is inserted in the socket). This effectively disables the fusing at functionality and forces the cartridge to function as a simple switch, so that the ringing voltage will be enabled or disabled by the insertion or removal of the fuse cartridge.
In a first, preferred embodiment of the invention, the socket is installed at a location that controls the enable signal of the ringing voltage generator, which prevents a hazardous voltage from being generated once the ring fuse cartridge is removed from its socket. It is also possible to place the fuse in series with the power source of the ringing voltage generator.
In a second embodiment of the invention, intended for use with an external ringing voltage generator, the socket is connected such that the load and alarm terminals are connected in common to the line card, while the source terminal is connected to the external ringing voltage generator. As in the first embodiment, the second embodiment also directly interrupts the ringing voltage when the ring fuse cartridge is removed from its socket. This embodiment is somewhat less desirable than the first, since the ringing voltage is actually presented to the fuse, and may violate safety requirements. Also, this approach only blocks the ringing voltage from reaching the line card. The source side of the fuse still has a ringing voltage present. Another technique to directly interrupt the ringing voltage (as in the case of an external source), without imposing a hazardous voltage on the fuse, is to have the fuse control a relay (which then interrupts the voltage).